This invention relates to an injectable packing formulation for packing a stuffing box assembly, and to the seals formed thereby. More particularly, this invention relates to an injectable packing formulation for packing the stuffing box assembly of pumps, valves, expansion joints, and like structures, comprising particles of flexible graphite and a suitable lubricant.
Known injectable packing formulations are usually composed of an oil and a filler material, such as asbestos and/or polytetrafluoroethylene. The oil present in such formulations serves two primary purposes. The first is to facilitate injection of the packing into the stuffing box assembly of a pump, valve or expansion joint by reducing the resistance to flow of the solid filler material. The second is to serve as the agent through which pressure is maintained in the stuffing box. Thus, because such liquid is essentially non-compressible, it readily transmits the pressure applied to it and thereby acts to maintain an effective seal. As such it forms an integral part of the seal and the performance of the packing rests first and foremost upon this liquid agent.
Unfortunately, many oils and other liquids employed in injectable packing formulations begin to chemically decompose and/or leak from the stuffing box in which they are employed at temperatures as low as 200.degree. F. At higher temperatures, e.g., from 400.degree. F. to 500.degree. F., the decomposition may yield residual deposits that are hard and abrasive, and damage to the stem or slide of the stuffing box may result from movement of these parts against these deposits. More importantly, however, any fluid loss sustained as a result of such leakage, or any void space created by the decomposition, causes a drop in internal pressure within the stuffing box. As a result, the seal fails and additional packing must be added to the system.
Another source of failure of seals packed with conventional injectable packing formulations is the plastic, putty-like nature of these materials. This results in the sealing off of air pockets within the mass of such formulations and the introduction of these pockets into the stuffing box along with the packing. Inevitably, the air escapes from the packing, the packing collapses into the void created, and the seal fails.
Still another source of failure of seals packed with conventional injectable packing formulations lies with the fillers employed in such formulations. Thus, certain conventional fillers, such as asbestos, are highly abrasive and require large quantities of lubricant to be injected into a stuffing box. However, as noted above, seal performance is adversely affected by the loss of pressure which occurs as a result of the decomposition and leakage of fluid which occurs at elevated temperatures. In addition, at temperatures of 450.degree. F. and above, asbestos itself undergoes chemical changes that causes it to shrink and harden. Not only does seal performance suffer by the void space created by this shrinkage, but the resulting hardening of the packing makes the addition of make-up packing more difficult.
Other fillers, such as polytetrafluoroethylene, undergo cold flow even at temperatures as low as room temperature. As a result, attempts to apply pressure to formulations containing such materials often results in forcing the packing back into the injection mechanism and/or through tolerance spaces between part interfaces. Furthermore, at temperatures of 450.degree. F. or more, polytetrafluoroethylene becomes highly plastic and decomposes leaving substantial voids. Not only is pressure within the stuffing box adversely affected by the voids created by the decomposition, but the plastic polytetrafluoroethylene begins to flow from the stuffing box along with the lubricant at such temperatures causing an even greater drop in pressure and seal performance.